Conventional puffer type gas circuit breakers are disclosed in, for example, U.S. Pat. No. 3,839,613, "Development of 240/300 kV, 50 kV 2,000 A, 4000 A, 8,000 A, 2-cycle Puffer Type SF.sub.6 gas Circuit Breakers", Hitachi Review 23 (1974), pages 343 to 352, and "Development of High Power 2 Cycle Puffer Type Gas Circuit Breakers", IEEE Conf. Paper C 74 089-9. A gas circuit breaker of such type is shown in FIGS. 1 and 2.
Gas Circuit Breaker 101, as shown in FIGS. 1 and 2, is disposed in a container (not shown) filled with an arc-extinguishing gas, such as SF.sub.6. The circuit breaker 101 comprises a fixed member 104, stationarily mounted with respect to the container, a fixed arc contactor 109, a main fixed contactor 110, and a movable member 112 including a main movable contactor 138 and a movable arc contactor 133 which is separable from the fixed arc contactor 109 in the axial direction of an arrow A so as to generate an arc 161 therebetween. A puffer chamber 130 is defined between a puffer cylinder 131 of the movable member 121 and a puffer piston 115 of a frame body 111 stationarily fixed with respect to the container. When the movable member 121 moves in the direction of the arrow A through an operating shaft member 124 of the movable member 121, due to a relative motion of the puffer piston 115 of the frame body 111 into the puffer chamber 130 in the direction of an arrow B, gas in the puffer chamber 130 is compressed and enters a chamber 190 defined in a a nozzle 142 made of an electrically insulating material through an opening 132 formed at one end of the puffer chamber 130. When the movable member 121 is further drawn out in the direction of the arrow A with respect to the fixed member 104 until the tip end of the fixed arc contactor 109 slips out of a small diameter throat portion 147 of the insulating nozzle 142 surrounding the tip ends of the contactors 109 and 133, the compressed gas in the chamber 190 flows through a region where the arc 161 is produced as a gas flow 162 passing through the throat portion 147 so as to cool the gaseous plasma of the arc 161. In this case, openings 139 of an exhaust passage 140 defined inside a shaft 191 of the movable member 121 are communicated with openings 120 formed in a cylindrical shaft portion 192 of the puffer piston 115, so that a gas flow 163 is formed simultaneously which is directed to flow from the chamber 190 and pass through the axial exhaust passage 140 and the openings 139 and 120. This gas flow 163 serves to cool the gaseous plasma of the arc 161. Consequently, double gas flows 162 and 163 effect cooling of the arc 161 to extinguish the arc 161, thereby interrupting the current between the fixed arc contactor 109 and the movable arc contactor 133.
However, in this kind of conventional gas circuit breaker 101, a larger force is required for operation of separating the movable contactor 133 in the direction of the arrow A. More particularly, since it is indispensable to compress the gas for generating an arc-extinguishing gas flow, this operating force could not be substantially reduced. Further, the diameter of the exhaust passage 140 can not be very large to avoid an increase in the diameter of the breaker although the exhaust passage 140 through the shaft 191 is long. Consequently, the flow resistance through the exhaust passage 140 is increased to hinder the gas from flowing sufficiently, resulting in a difficulty in extinguishing the arc 161.
Additionally in, for example, Japanese Patent Laid-Open Publication No. 53-117758 a thermal puffer type gas circuit breaker comprises an expansion or arc-extinguishing chamber for compressing gas using heat of the arc and extinguishes an arc by blowing or puffing the gas compressed in the expansion chamber against the arc (i.e. by flowing the gas along the arc to cool the arc). However, in this thermal puffer type gas circuit breaker as well, although the double flow method is adopted by providing in the exhaust passage a pressure-responsive valve utilizing spring force with the intention of interrupting a large electric current as well, it is hard to stably extinguish the arc for a long time period over a wide range of electric current due to presence of the pressure-responsive valve or the like, and a lower performance for interrupting a small electric current is experienced due to the presence of the pressure-responsive valve.